v$open_cursor与session_cached_cursor

v$open_cursor与session_cached_cursor

v$open_cursor是oracle提供的一张查看每个session打开过的cursor的视图,它的表结构如下
SQL> desc v$open_cursor
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 SADDR                                                          RAW(4)
 SID                                                            NUMBER
 USER_NAME                                                      VARCHAR2(30)
 ADDRESS                                                        RAW(4)
 HASH_VALUE                                                     NUMBER
 SQL_TEXT                                                       VARCHAR2(60)

当我们执行一条sql语句的时候,我们将会在shared pool产生一个library cache object,cursor就是其中针对于sql语句的一种library cache object.另外我们会在pga有一个cursor的拷贝,同时在客户端会有一个statement handle,这些都被称为cursor,在v$open_cursor里面我们可以看到当前打开的cursor和pga内cached cursor.

session_cached_cursor
这个参数限制了在pga内session cursor cache list的长度,session cursor cache list是一条双向的lru链表,当一个session打算关闭一个cursor时,如果这个cursor的parse count超过3次,那么这个cursor将会被加到session cursor cache list的MRU端.当一个session打算parse一个sql时,它会先去pga内搜索session cursor cache list,如果找到那么会把这个cursor脱离list,然后当关闭的时候再把这个cursor加到MRU端.session_cached_cursor提供了快速软分析的功能,提供了比soft parse更高的性能.

下面做个实验来证明以上这些理论

SQL> select distinct sid from v$mystat;

       SID
----------
        12


SQL> show parameter session_cached_cursors

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
session_cached_cursors               integer     0

这是设置session_cached_cursors为0,禁止了cache cursor的功能

我们另开一个sqlplus来看v$open_cursor

SQL> select * from v$open_cursor where sid=12;

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99DA7F20  745629022 select distinct sid from v$mystat

能在v$open_cursor里看到这条sql

执行下一条语句
SQL> select owner from test where rownum=1;

OWNER
------------------------------
SYS

在来看v$open_cursor有什么变化

SQL> /

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99C9AEF8  608211958 select owner from test where rownum=1

可以看到语句变了,来看看current open cursor和session cached cursor的数目
SQL>  select sid,n.name,s.value  used
  2   from
  3     sys.v_$statname  n,
  4     sys.v_$sesstat  s
  5   where
  6     n.name in ('opened cursors current', 'session cursor cache count') and
  7     s.statistic# = n.statistic#
  8     and sid=12;

       SID NAME                                                                   USED
---------- ---------------------------------------------------------------- ----------
        12 opened cursors current                                                    1
        12 session cursor cache count                                                0

我们来把session_cached_cursors修改成1
SQL> alter session set session_cached_cursors=1;

Session altered.

再执行上面的语句

SQL> select owner from test where rownum=1;

OWNER
------------------------------
SYS

看看v$open_cursor

SQL> select * from v$open_cursor where sid=12;

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99C9AEF8  608211958 select owner from test where rownum=1

 

此处执行3次 select * from v$open_cursor where sid=12,oracle进行了3次soft parse
SQL> select owner from test where rownum=1;

OWNER
------------------------------
SYS
SQL> select owner from test where rownum=1;

OWNER
------------------------------
SYS

然后再执行

SQL> select distinct sid from v$mystat;

       SID
----------
        12

再来看看v$open_cursor有什么变化
SQL> /

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99C9AEF8  608211958 select owner from test where rownum=1
95D5AFCC         12 TAOBAO                         99DA7F20  745629022 select distinct sid from v$mystat

v$open_cursor有了两条记录,再看一下current open cursor和session cached cursor的数目

SQL>  select sid,n.name,s.value  used
  2   from
  3     sys.v_$statname  n,
  4     sys.v_$sesstat  s
  5   where
  6     n.name in ('opened cursors current', 'session cursor cache count') and
  7     s.statistic# = n.statistic#
  8     and sid=12;

       SID NAME                                                                   USED
---------- ---------------------------------------------------------------- ----------
        12 opened cursors current                                                    1
        12 session cursor cache count                                                1
       
果然有一句sql进了session cursor cache,"select owner from test where rownum=1"这个sql相关的cursor就被cache起来了.

再执行一条新的语句
SQL> select owner from test where rownum<2;

OWNER
------------------------------
SYS

看一下v$open_cursor
SQL> select * from v$open_cursor where sid=12;

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99CA4FFC 2584499260 select owner from test where rownum<2
95D5AFCC         12 TAOBAO                         99C9AEF8  608211958 select owner from test where rownum=1

可以看到上一次的"select distinct sid from v$mystat"已经被关闭,在v$open_cursor中不可见,但是cached cursor "select owner from test where rownum=1"还能看到

这时候运行3次select distinct sid from v$mystat
SQL> select distinct sid from v$mystat;

       SID
----------
        12
SQL> select distinct sid from v$mystat;

       SID
----------
        12
SQL> select distinct sid from v$mystat;

       SID
----------
        12
       
然后再运行select owner from test where rownum<2

SQL>  select owner from test where rownum<2;

OWNER
------------------------------
SYS


再去查看v$open_cursor

SQL> select * from v$open_cursor where sid=12;

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99C6C628 1755089113 select owner from test where rownum<2
95D5AFCC         12 TAOBAO                         99DA7F20  745629022 select distinct sid from v$mystat

发现原来cached cursor"select owner from test where rownum=1"已经被替换出了session cursor cache,而变成了"select distinct sid from v$mystat"

我们再修改session_cached_cursors

alter session set session_cached_cursors=2;

再重复一下上面的过程,再去看v$open_cursor

SQL> /

SADDR           SID USER_NAME                      ADDRESS  HASH_VALUE SQL_TEXT
-------- ---------- ------------------------------ -------- ---------- ------------------------------------------------------------
95D5AFCC         12 TAOBAO                         99CA4FFC 2584499260 select owner from test where rownum<2
95D5AFCC         12 TAOBAO                         99C9AEF8  608211958 select owner from test where rownum=1
95D5AFCC         12 TAOBAO                         99DA7F20  745629022 select distinct sid from v$mystat

可以看到现在有3个cursor,看一下current open cursor和session cached cursor的数目


SQL>  select sid,n.name,s.value  used
  2   from
  3     sys.v_$statname  n,
  4     sys.v_$sesstat  s
  5   where
  6     n.name in ('opened cursors current', 'session cursor cache count') and
  7     s.statistic# = n.statistic#
  8     and sid=12;

       SID NAME                                                                   USED
---------- ---------------------------------------------------------------- ----------
        12 opened cursors current                                                    1
        12 session cursor cache count                                                2
       
这下有2个cached cursor,一个current open cursor.

从这个实验我们可以清楚得看到v$open_cursor视图和session_cached_cursors参数的作用.

在这里再提一下另一个与cursor有关的参数open_cursors,这个参数限制了每个session可以打开的最多cursor,如果当前打开的cursor数超过了open_cursor就会报ORA-01000: maximum open cursors exceeded,通常在使用java connection pool时如果处理不当将会导致ORA-01000,如果java应用服务器端使用软关闭cursor,并保留这些cursor在在cache size,那么数据库端认为这个cursor还是打开的,.所以在open_cursors,session_cached_cursors等参数时一定要和应用端的cursor cache size等参数搭配好.

posted on 2009-08-27 00:17  一江水  阅读(3166)  评论(0编辑  收藏  举报